Causal consistency concept in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding causal consistency in MongoDB

Which statement best describes causal consistency in MongoDB?

A Operations that are causally related are seen by clients in the same order, but unrelated operations may be seen in different orders.

B Operations are only visible to the client that performed them until a manual refresh.

C Operations are seen by all clients in the exact order they were performed globally.

D All operations are immediately visible to all clients without delay.

💡 Hint

Think about how related changes should appear in order to maintain logical flow.

Causal consistency ensures that if one operation depends on another, all clients see them in that order. Unrelated operations may appear in different orders.

❓ query_result

intermediate

Effect of causal consistency on read operations

Given a MongoDB replica set with causal consistency enabled, if a client writes a document and then reads it immediately, what will the read operation return?

A The read will return null until the write is acknowledged by all replicas.

B The read may return an older version if the write has not propagated yet.

C The read will fail with a consistency error.

D The read will always return the latest written document.

💡 Hint

Consider what causal consistency guarantees about visibility of your own writes.

Causal consistency guarantees that a client will always see its own writes immediately on subsequent reads.

📝 Syntax

advanced

Enabling causal consistency in MongoDB sessions

Which of the following code snippets correctly enables causal consistency for a MongoDB session in Node.js?

MongoDB

const session = client.startSession({ /* options here */ });

A const session = client.startSession({ enableCausalConsistency: true });

B const session = client.startSession({ causal_consistency: true });

C const session = client.startSession({ causalConsistency: true });

D const session = client.startSession({ causalConsistencyEnabled: true });

💡 Hint

Check the exact option name for enabling causal consistency in the MongoDB Node.js driver.

The correct option name is causalConsistency with camelCase and boolean true.

❓ optimization

advanced

Optimizing read preference with causal consistency

When using causal consistency in MongoDB, which read preference setting optimizes for reading your own writes while allowing reads from secondary replicas?

A Read preference set to <code>primary</code> only.

B Read preference set to <code>primaryPreferred</code>.

C Read preference set to <code>secondaryPreferred</code>.

D Read preference set to <code>secondary</code> only.

💡 Hint

Think about a setting that prefers primary but can fall back to secondary.

primaryPreferred allows reading from the primary to see your writes but can read from secondaries if primary is unavailable.

🔧 Debug

expert

Diagnosing causal consistency violation in MongoDB

You observe that a client sometimes reads stale data that does not reflect its previous writes, despite using causal consistency sessions. Which is the most likely cause?

A The client is not using the same session for the write and subsequent read operations.

B The MongoDB cluster does not support causal consistency.

C The write concern is set to <code>w: 0</code> causing writes to be unacknowledged.

D The client is using read preference <code>primary</code> instead of <code>secondary</code>.

💡 Hint

Consider how sessions relate to causal consistency guarantees.

Causal consistency requires using the same session for related operations to maintain order and visibility.